2009-10 CEGEG046 / GEOG3051
Principles & Practice of Remote Sensing (PPRS)
8: RADAR 1
Dr. Mathias (Mat) Disney
UCL Geography
Office: 113, Pearson Building
Tel: 7670 05921
Email: mdisney@ucl.geog.ac.uk
www.geog.ucl.ac.uk/~mdisney
OVERVIEW AGENDA
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Principles of RADAR, SLAR and SAR
Characteristics of RADAR
SAR interferometry
Applications of SAR
Student summaries
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LECTURE 1
PRINCIPLES AND CHARACTERISTICS OF
RADAR, SLAR AND SAR
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Examples
Definitions
Principles of RADAR and SAR
Resolution
Frequency
Geometry
Radiometry
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9/8/91
ERS-1 (11.25 am), Landsat (10.43 am)
4
The image at the top
was acquired through
thick cloud cover by the
Spaceborne Imaging
Radar-C/X-band
Synthetic Aperture
Radar (SIR-C/X-SAR)
aboard the space
shuttle Endeavour on
April 16, 1994.
The image on the
bottom is an optical
photograph taken by the
Endeavour crew under
clear conditions
during the second flight
of SIR-C/X-SAR on
October 10, 1994
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Ice
6
Oil slick
Galicia, Spain
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Nicobar Islands
December
2004
tsunami
flooding in
red
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Paris
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Definitions
• Radar - an acronym for Radio Detection And Ranging
• SLAR – Sideways Looking Airborne Radar
– Measures range to scattering targets on the ground, can be used
to form a low resolution image.
• SAR Synthetic Aperture Radar
– Same principle as SLAR, but uses image processing to create
high resolution images
• IfSAR Interferometric SAR
– Generates X, Y, Z from two SAR images using principles of
interferometry (phase difference)
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References
• Henderson and Lewis, Principles and Applications of Imaging Radar,
John Wiley and Sons
• Allan T D (ed) Satellite microwave remote sensing, Ellis Horwood,
1983
• F. Ulaby, R. Moore and A. Fung, Microwave Remote Sensing: Active
and Passive (3 vols), 1981, 1982, 1986
• S. Kingsley and S. Quegan, Understanding Radar Systems, SciTech
Publishing.
• C. Oliver and S. Quegan, Understanding Synthetic Aperture Radar
Images, Artech House, 1998.
• Woodhouse I H (2000) Tutorial review. Stop, look and listen: auditory
perception analogies for radar remote sensing, International Journal of
Remote Sensing 21 (15), 2901-2913.
• Jensen, J. R. (2000) Remote sensing of the Environment, Chapter 9.
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Web sites
Canada
• http://www.ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter
3/01_e.php
• ftp://ftp2.ccrs.nrcan.gc.ca/ftp/ad/MAS/fundamentals_e.pdf
ESA
• http://earth.esa.int/applications/data_util/SARDOCS/space
borne/Radar_Courses/
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What is RADAR?
• Radio Detection and Ranging
• Radar is a ranging instrument
• (range) distances inferred from time elapsed between
transmission of a signal and reception of the returned
signal
• imaging radars (side-looking) used to acquire images
(~10m - 1km)
• altimeters (nadir-looking) to derive surface height
variations
• scatterometers to derive reflectivity as a function of
incident angle, illumination direction, polarisation, etc
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What is RADAR?
• A Radar system has three primary functions:
- It transmits microwave (radio) signals towards a
scene
- It receives the portion of the transmitted energy
backscattered from the scene
- It observes the strength (detection) and the time
delay (ranging) of the return signals.
• Radar provides its own energy source and, therefore,
can operate both day or night. This type of system is
known as an active remote sensing system.
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Principle of RADAR
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Principle of
ranging and
imaging
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Radar Pulse
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ERS 1 and 2
geometry
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Radar wavelength
• Most remote sensing radars operate at wavelengths
between 0.5 cm and 75 cm:
X-band: from 2.4 to 3.75 cm (12.5 to 8 GHz).
C-band: from 3.75 to 7.5 cm (8 to 4 GHz).
S-band: from 7.5 to 15 cm (4 to 2 GHz).
L-band: from 15 to 30 cm (2 to 1 GHz).
P-band: from 30 to 100 cm (1 to 0.3 GHz).
• The capability to penetrate through precipitation or
into a surface layer is increased with longer
wavelengths. Radars operating at wavelengths > 2 cm
are not significantly affected by cloud cover. Rain
does become a factor at wavelengths < 4 cm.
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Comparison of C band and L band SAR
C-band
L-band
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Choice of wave length
• Radar wavelength should be matched to the size of
the surface features that we wish to discriminate
• – e.g. Ice discrimination, small features, use X-band
• – e.g. Geology mapping, large features, use L-band
• – e.g. Foliage penetration, better at low frequencies,
use P-band
• In general, C-band is a good compromise
• New airborne systems combine X and P band to give
optimum measurement of vegetation
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Synthetic Aperture Radar (SAR)
• Imaging side-looking accumulates data along path –
ground surface “illuminated” parallel and to one side
of the flight direction. Data, processing is needed to
produce radar images.
• The across-track dimension is the “range”. Near range
edge is closest to nadir; far range edge is farthest
from the radar.
• The along-track dimension is referred to as “azimuth”.
• Resolution is defined for both the range and azimuth
directions.
• Digital signal processing is used to focus the image
and obtain a higher resolution than achieved by
conventional radar
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Principle of
Synthetic
Aperture Radar
SAR
Doppler
frequency due to
sensor
movement
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Azimuth resolution: synthetic aperture
v

R
Target
time spent in beam = arc length / v =
Ry / v = Rl / vLa
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Resolution
τ
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Range and azimuth resolution (RAR)
Range resolution (across track)
Azimuth resolution (along track)
Sl
Tc
Rr 
2 cos 
Ra 
T = duration of the radar pulse
c = speed of light
γ = depression angle
L = antenna length
S = slant range = height/sin
λ = wavelength
L
cos : inverse relationship with angle
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Resolution of SAR
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Important point
• Resolution cell (i.e. the cell defined by the resolutions
in the range and azimuth directions) does NOT mean
the same thing as pixel. Pixel sizes need not be the
same thing. This is important since (i) the
independent elements in the scene are resolutions
cells, (ii) neighbouring pixels may exhibit some
correlation.
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Some Spaceborne Systems
Launch
Agency
properties
ERS-1
ERS-2
Radarsat
1991 (-1997)
1995
1995
ESA
C-VV
CSA
C-HH
JERS
1992-1998
NASDA
L-HH
NASA
DARA / ASI
L,C, X
polarimetric
SIR-C/X-SAR 1994 (2x10 days)
resolution
swath
25 m
100 km
10-100 m
40-500 km
18 m
76 km
30 m
15-90 km
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ERS 1 and 2 Specifications
Geometric specifications
Spatial resolution:
along track <=30 m
across-track <=26.3 m
Swath width:
102.5 km (telemetered)
80.4 km (full performance)
Swath standoff: 250 km to the right of the satellite
track
Localisation accuracy:
along track <=1 km;
across-track <=0.9 km
Incidence angle: near swath 20.1deg.
mid swath 23deg.
far swath 25.9deg
Incidence angle tolerance:
<=0.5 deg.
Radiometric specifications:
Frequency:
5.3 GHz (C-band)
Wave length:
5.6 cm
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Speckle
• Speckle appears as
“noisy” fluctuations in
brightness
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Speckle
• Fading and speckle are the inherent “noise-like” processes which
degrade image quality in a coherent imaging system.
• Local constructive and destructive interference appears in the
image as bright and dark speckles, respectively.
• Using independent data sets to estimate the same ground patch,
by average independent samples, can effectively reduce the
effects of speckle. This can be done by:
• Multiple-look filtering, separates the maximum synthetic aperture
into smaller sub-apertures generating independent looks at
target areas based on the angular position of the targets.
Therefore, looks are different Doppler frequency bands.
• Averaging (incoherently) adjacent pixels.
• Reducing these effects enhances radiometric resolution at the
expense of spatial resolution.
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Speckle
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Speckle
• Radar images are formed coherently and
therefore inevitably have a “noise-like”
appearance
• Implies that a single pixel is not representative of
the backscattering
• “Averaging” needs to be done
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Multi-looking
• Speckle can be suppressed by “averaging” several
intensity images
• This is often done in SAR processing
• Split the synthetic aperture into N separate parts
• Suppressing the speckle means decreasing the width
of the intensity distribution
• We also get a decrease in spatial resolution by the
same factor (N)
• Note this is in the azimuth direction (because it
relies on the motion of the sensor which is in this
direction)
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Speckle
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Principle of
ranging and
imaging
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Geometric effects
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Shadow
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Foreshortening
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Layover
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Layover
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Los
Angeles
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Radiometric aspects – the RADAR equation
• The brightness of features in an image is usually a
combination of several variables. We can group these
characteristics into three areas which fundamentally
control radar energy/target interactions. They are:
– Surface roughness of the target
– Radar viewing and surface geometry relationship
– Moisture content and electrical properties of the target
• http://earth.esa.int/applications/data_util/SARDOCS/sp
aceborne/Radar_Courses/Radar_Course_III/radar_equ
ation.htm
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Returned energy
• Angle of the surface to the incident radar beam
– Strong from facing areas, weak from areas facing away
• Physical properties of the sensed surface
– Surface roughness
– Dielectric constant
Smooth
Rough
– Water content of the surface
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Roughness
Smooth, intermediate or rough?
• Peake and Oliver (XX) – surface height variation h
–
–
–
–
Smooth: h < l/25sin β
Rough: h > l/4.4sin β
Intermediate
β is depression angle, so depends on l AND imaging
geometry
http://rst.gsfc.nasa.gov/Sect8/Sect8_2.html
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Oil slick
Galicia, Spain
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Los
Angeles
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Source: Graham 2001
Response to soil moisture
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Crop moisture
SAR image
In situ irrigation
Source: Graham 2001
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Types of
scattering of
radar from
different
surfaces
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Scattering
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The Radar Equation
The fundamental relation between the characteristics of the radar, the target,
and the received signal is called the radar equation. The geometry of scattering
from an isolated radar target (scatterer) is shown.
When a power Pt is transmitted by an antenna with gain Gt , the power per unit
solid angle in the direction of the scatterer is Pt Gt, where the value of Gt in that
direction is used.
READ:http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_C
ourses/Radar_Course_III/radar_equation.htm and Jensen Chapter 9
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The Radar Equation
We may rewrite the radar equation as two alternative forms, one in
terms of the antenna gain and the other in terms of the antenna
area
Because
R = range
P = power
G = gain of antenna
A = area of the antenna
Where:
The Radar scattering cross section
The cross-section σ is a function of the directions of the incident wave and the
wave toward the receiver, as well as that of the scatterer shape and dielectric
properties.
fa is absorption
Ars is effective area of incident beam received by scatterer
READ:
Gts is gain of the scatterer in the direction of the receiver
http://earth.esa.int/applications/data_util/SARDOCS/spaceborne/Radar_Cour
ses/Radar_Course_III/radar_equation.htm
And Jensen Chapter 9
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Measured quantities
s2
• Radar cross section [dBm2]
• Bistatic scattering coefficient [dB]
• Backscattering coefficient [dB]
|E |
lim
2
  r   4r
i2
|E |
s2
2
lim 4r |E |
0
 r
i2
A cos
i |E |
s2
2
lim 4r |E |
0
 r
i2
A
|E |
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The Radar Equation: Point targets
• Power received
P PG
r
t t
1
4R
2

1
4R
A
2 r
• Gt is the transmitter gain, Ar is the effective area of
receiving antenna and  the effective area of the target.
Assuming same transmitter and receiver, A/G=l2/4
2 2
l G
P P

3 4
r
t
(4 ) R
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Calibration of SAR
• Emphasis is on radiometric calibration to
determine the radar cross section
• Calibration is done in the field, using test sites
with transponders.
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